System and method of producing foamed cement in a laboratory environment

ABSTRACT

Systems and methods related to preparing foamed cement for laboratory analysis are provided. A prepared cement slurry is placed in a cement reservoir cell configured to pressurize the cement slurry contained within the cement reservoir cell to a capture pressure. After pressurization, the cement slurry and a compressed gas are introduced into a foam generator. Foamed cement generated in the foam generator is introduced from the tee into a foam capture cell where it can cure prior to analysis.

FIELD

This disclosure relates to systems and methods for producing foamedcement as used in oil and gas wells. More specifically, it relates tolaboratory systems and methods of producing foamed cement to emulatefoamed cement produced at the well site.

BACKGROUND

Cementing has been used in oil well drilling since the early 20^(th)century. It has become a very important factor to achieving goodintegrity of an oil well. Foamed cement is a lightweight cement that hasbecome an important solution for zonal isolation in oil and gas wells.It provides a variety of benefits over the traditional cement systems.Enhanced mechanical properties, improved strength-to-density ratio,improved mud displacement, improved bonding with casing and formation,reduced gas migration risk, and improved long-term well integrity aresome of the advantages.

Laboratory testing is an essential contributor to successful fieldresults in oil well cementing operations. Laboratory testing provides anindication of how the cement will behave when used in the field.Unfortunately, often the properties of foamed cement produced inlaboratory conditions do not seem to match with the actual results inthe field, which may lead to less than optimum foam-cement designs. Thiscan be because the method used to generate foamed cement in thelaboratory is fundamentally different from that in the field.Accordingly, laboratory generated foamed cement that more closelyparallels field generated foamed cement would be a great aid indesigning optimum foamed cements for specific wells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for preparation and delivery of a cementcomposition to a wellbore.

FIG. 2A illustrates surface equipment that may be used in placement of acement composition in a wellbore.

FIG. 2B illustrates placement of a cement composition into a wellboreannulus.

FIG. 3 is a chart of gas-bubble size distribution of foamed cementproduced with a Waring blender according to API 10B-4 and produced withfield equipment at a well site.

FIG. 4 is a schematic view of a pressurized laboratory foam-cement testsystem in accordance with aspects of the current disclosure.

FIG. 5 is a schematic view of a foam generator suitable for use in thetest system of FIG. 4.

FIG. 6 is a chart of gas-bubble size versus cumulative distribution byvolume for foamed cement samples produced with different methods andapparatuses.

FIG. 7 is a chart of gas-bubble size versus total volume fraction forthe same foamed cement samples as FIG. 6.

DETAILED DESCRIPTION

The present disclosure may be understood more readily by reference tothe following detailed description as well as to the examples includedtherein. In addition, numerous specific details are set forth in orderto provide a thorough understanding of the embodiments described herein.However, those of ordinary skill in the art will understand that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures and components have notbeen described in detail so as not to obscure the related relevantfeature being described. Additionally, the description is not to beconsidered as limiting the scope of the embodiments described herein.

Referring now to the drawings, wherein like reference numbers are usedherein to designate like elements throughout the various views, variousembodiments are illustrated and described. The figures are notnecessarily drawn to scale, and in some instances the drawings have beenexaggerated and/or simplified in places for illustrative purposes only.In the following description, the terms “upper,” “upward,” “lower,”“below,” and the like, are used for the embodiments illustrated;however, one skilled in the art will realize that generally theapparatus or parts thereof described can be in other orientations. Theterms “inwardly” and “outwardly” are directions toward and away from,respectively, the geometric center of a referenced object. Wherecomponents of relatively well-known designs are employed, theirstructure and operation will not be described in detail. One of ordinaryskill in the art will appreciate the many possible applications andvariations of the present invention based on the following description.

As indicated above, cementing in oil and gas wells is an importantfactor to achieve good integrity of the well. Cement, including foamedcement, may directly or indirectly affect one or more components orpieces of equipment associated with the preparation, delivery,recapture, recycling, reuse, and/or disposal of the disclosed cementcompositions. For example, cement compositions may directly orindirectly affect one or more mixers, related mixing equipment, mudpits, storage facilities or units, composition separators, heatexchangers, sensors, gauges, pumps, compressors, and the like usedgenerate, store, monitor, regulate, and/or recondition the exemplarycement compositions. Cement compositions may also directly or indirectlyaffect any transport or delivery equipment used to convey the cementcompositions to a well site or downhole such as, for example, anytransport vessels, conduits, pipelines, trucks, tubulars, and/or pipesused to compositionally move the cement compositions from one locationto another, any pumps, compressors, or motors (e.g., topside ordownhole) used to drive the cement compositions into motion, any valvesor related joints used to regulate the pressure or flow rate of thecement compositions, and any sensors (i.e., pressure and temperature),gauges, and/or combinations thereof, and the like. Cement compositionsmay also directly or indirectly affect the various downhole equipmentand tools that may come into contact with the cementcompositions/additives such as, but not limited to, wellbore casing,wellbore liner, completion string, insert strings, drill string, coiledtubing, slickline, wireline, drill pipe, drill collars, mud motors,downhole motors and/or pumps, cement pumps, surface-mounted motorsand/or pumps, centralizers, turbolizers, scratchers, floats (e.g.,shoes, collars, valves, etc.), logging tools and related telemetryequipment, actuators (e.g., electromechanical devices, hydromechanicaldevices, etc.), sliding sleeves, production sleeves, plugs, screens,filters, flow control devices (e.g., inflow control devices, autonomousinflow control devices, outflow control devices, etc.), couplings (e.g.,electro-hydraulic wet connect, dry connect, inductive coupler, etc.),control lines (e.g., electrical, fiber optic, hydraulic, etc.),surveillance lines, drill bits and reamers, sensors or distributedsensors, downhole heat exchangers, valves and corresponding actuationdevices, tool seals, packers, cement plugs, bridge plugs, and otherwellbore isolation devices, or components, and the like.

Turning now to the figures, FIG. 1 illustrates a system 2 forpreparation of a cement composition and delivery to a wellbore as mightbe done at a field site. As shown, the cement composition may be mixedin mixing equipment 4, such as a jet mixer, re-circulating mixer, or abatch mixer, for example, and then pumped via pumping equipment 6 to thewellbore. In some embodiments, the mixing equipment 4 and the pumpingequipment 6 may be disposed on one or more cement trucks as will beapparent to those of ordinary skill in the art. In some embodiments, ajet mixer may be used, for example, to continuously mix the composition,including water, as it is being pumped to the wellbore.

An example technique and system for placing a cement composition into asubterranean formation will now be described with reference to FIGS. 2Aand 2B. FIG. 2A illustrates surface equipment 10 that may be used inplacement of a cement composition in accordance with certainembodiments. It should be noted that while FIG. 2A generally depicts aland-based operation, those skilled in the art will readily recognizethat the principles described herein are equally applicable to subseaoperations that employ floating or sea-based platforms and rigs, withoutdeparting from the scope of the disclosure. As illustrated by FIG. 2A,the surface equipment 10 may include a cementing unit 12, which mayinclude one or more cement trucks. The cementing unit 12 may includemixing equipment 4 and pumping equipment 6 (e.g., FIG. 1) as will beapparent to those of ordinary skill in the art. The cementing unit 12may pump a cement composition 14 through a feed pipe 16 and to acementing head 18 which conveys the cement composition 14 downhole. Whenfoamed cement is used, the surface equipment may include a gas deliveryand foam generating system 17 for delivering and mixing a gas, such asnitrogen, into the cement composition.

Turning now to FIG. 2B, the cement composition 14 may be placed into asubterranean formation 20 in accordance with example embodiments. Asillustrated, a wellbore 22 may be drilled into the subterraneanformation 20. While wellbore 22 is shown extending generally verticallyinto the subterranean formation 20, the principles described herein arealso applicable to wellbores that extend at an angle through thesubterranean formation 20, such as horizontal and slanted wellbores. Asillustrated, the wellbore 22 comprises walls 24. In the illustratedembodiments, a surface casing 26 has been inserted into the wellbore 22.The surface casing 26 may be cemented to the walls 24 of the wellbore 22by cement sheath 28. In the illustrated embodiment, one or moreadditional conduits (e.g., intermediate casing, production casing,liners, etc.) shown here as casing 30 may also be disposed in thewellbore 22. As illustrated, there is a wellbore annulus 32 formedbetween the casing 30 and the walls 24 of the wellbore 22 and/or thesurface casing 26. One or more centralizers 34 may be attached to thecasing 30, for example, to centralize the casing 30 in the wellbore 22prior to and during the cementing operation.

With continued reference to FIG. 2B, the cement composition 14 may bepumped down the interior of the casing 30. The cement composition 14 maybe allowed to flow down the interior of the casing 30 through the casingshoe 42 at the bottom of the casing 30 and up around the casing 30 intothe wellbore annulus 32. The cement composition 14 may be allowed to setin the wellbore annulus 32, for example, to form a cement sheath thatsupports and positions the casing 30 in the wellbore 22. While notillustrated, other techniques may also be utilized for introduction ofthe cement composition 14. By way of example, reverse circulationtechniques may be used that include introducing the cement composition14 into the subterranean formation 20 by way of the wellbore annulus 32instead of through the casing 30.

As it is introduced, the cement composition 14 may displace other fluids36, such as drilling fluids and/or spacer fluids that may be present inthe interior of the casing 30 and/or the wellbore annulus 32. At least aportion of the displaced fluids 36 may exit the wellbore annulus 32 viaa flow line 38 and be deposited, for example, in one or more retentionpits 40 (e.g., a mud pit), as shown on FIG. 2A. Referring again to FIG.2B, a bottom plug 44 may be introduced into the wellbore 22 ahead of thecement composition 14, for example, to separate the cement composition14 from the fluids 36 that may be inside the casing 30 prior tocementing. After the bottom plug 44 reaches the landing collar 46, adiaphragm or other suitable device ruptures to allow the cementcomposition 14 through the bottom plug 44. In FIG. 2B, the bottom plug44 is shown on the landing collar 46. In the illustrated embodiment, atop plug 48 may be introduced into the wellbore 22 behind the cementcomposition 14. The top plug 48 may separate the cement composition 14from a displacement fluid 50 and also push the cement composition 14through the bottom plug 44.

The above generally describes cementing operations in a well; however,in order to ensure that foamed cement meets the needs dictated by thewell conditions, laboratory testing will usually be performed. Suchlaboratory testing is an essential contributor to successful fieldresults in oil well cementing operations. Traditionally, laboratoryanalysis of foamed cement is carried out according to API 10B-4. Thisstandard test method provides for generating foamed cement by mixing airand cement in a multi-blade Waring blender under atmospheric pressureconditions. In the field, foamed cement is generally produced by mixinga stream of high-pressure nitrogen gas with a stream of cement slurrythrough a jet-mixing nozzle called a “foam generator” under the wellheadpressure. Differences in traditional laboratory foam-cement generationand field foam-cement generation can be seen by reference to FIG. 3.From FIG. 3, it can be seen that the gas bubble size of the lab foamsample has a very narrow distribution range at lower foam quality, whichbecomes much wider as foam quality increases. Moreover, the median gasbubble size of the lab foam sample increases significantly withincreasing foam quality. On the other hand, the gas-bubble sizedistribution of field foam sample shows very little dependency on foamquality.

“Foam quality” describes the volume of gas relative to the total volumeof foam. For instance, a 20% quality implies that there is a volume of20% gas in the total volume of foam. Similarly, a 40% quality impliesthat there is a volume of 40% gas in the total volume of foam. Usuallythe range of foam quality for foamed cements is from about 15% to about30%, and more typically from 18% to 28%, because a higher quality cancause coalescence of the bubbles and cause excessive permeability.

As will be realized by an analysis of FIG. 3, the traditional laboratorytesting procedure does not produce a foamed cement representative offoamed cement produced in the field. Foamed cement produced bytraditional laboratory testing procedures has a significantly loweramount of smack bubbles (60 μm or less) than that produced in the field.The gas-bubble size range and distribution are also quite differentbetween traditional laboratory foamed cement and field foamed cement.Thus, due to the differences in morphology caused by the difference inbubble diameter and concentration, the properties of foamed cementdetermined by traditional lab methods may not be representative of theactual field conditions. In particular, as foam stability has a strongdependence on gas bubble size, current lab tests cannot reliablydetermine the stability of foamed cement produced in the field.

Turning now to FIG. 4, a system or apparatus 100 suitable for preparingfoamed cement for laboratory analysis in accordance with this disclosureis illustrated. Apparatus 100 generally comprises a source ofpressurized gas 102, a cement reservoir cell 110, a foam generator 124and foam capture cell 136. Pressurized gas source 102 can be anysuitable source, such as a compressor and/or a canister of pressurizedgas. Generally, nitrogen is preferred as the gas because nitrogen isinert and standardly used at the well site. However, the gas should beselected to match what will be used at the well site and can benitrogen, air, hydrogen or another suitable gas.

Pressurized gas source 102 is in fluid flow communication with foamgenerator tee 124 via conduit 104; thus, pressurized gas frompressurized gas source 102 is introduced to foam generator 124 throughconduit 104. Generally, the conduits used in apparatus 100 can bestainless steel tubing. Conduit 104 can include a pressure regulator 106and a check valve 108, or similar backflow prevention device.

Cement reservoir cell 110 is configured to hold a cement slurry until itis introduced to foam generator 124. Typically, cement reservoir cell110 can hold the cement slurry at an elevated pressure. As shown, cementreservoir cell 110 comprises a cylinder 112 and a piston 114. The upperend 116 of cement reservoir cell 110 is connected to a pump 118 suchthat pressure can be applied to a first side 113 of piston 114; thus, asthe pressure is applied, piston 114 will apply pressure to cement slurryon a second side 115 of piston 114 and contained in cylinder 112. Thus,cement reservoir cell 110 is configured to pressurize a cement slurrycontained within cylinder 112.

Cement reservoir cell 110 is in fluid flow communication with foamgenerator 124 via conduit 120, which is connected to cement reservoircell 110 at lower end 117. When valve 122 is closed, the cement slurrycan be pressurized to or slightly above the capture pressure. When thecement slurry has been pressurized, valve 122 can be opened to allowflow (driven by pump 118) into foam generator 124. The “capturepressure” is an adjustable parameter for the generation of thelaboratory foamed cement. Typically, the capture pressure is selected tocorrespond with the wellhead pressure in the field operations for whichthe foamed cement will be used. In this way, the foamed cement generatedin the laboratory by apparatus 100 is generated at a correspondingpressure to the foam generation at the well site.

As can be seen from FIG. 5, foam generator 124 may have variousconfigurations. Generally, its configuration will be similar to the foamgenerator used at the well site; thus, foam generator 124 could have atee design, a cross design, or another suitable design. As illustrated,foam generator 124 is a foam generator tee and comprises a first leg126, a second leg 128 and a third leg 130. First leg 126 is in fluidflow communication with conduit 104, and hence, with pressurized gassource 102. First leg 126 contains a choke 132, which controls the rateof gas fed into the foam generator 124 and provides a pressure drop toincrease the velocity of the gas. The pressure drop leads to asignificant increase in the kinetic energy of the gas, which suppliessufficient mixing energy to produce stable foamed cement. Typically,choke 132 will be an exchangeable choke; that is, choke 132 can beeasily replaced with alternative chokes to provide for different ratesand/or pressure reductions to accommodate different foamed cementqualities.

Second leg 128 is in fluid flow communication with conduit 120, andhence, with cement reservoir cell 110. As can be seen from FIG. 5, firstleg 126 and second leg 128 are perpendicular to each other; thus, gasentering first leg 126 and cement slurry entering second leg 128 mix atintersection 134 to produce foamed cement. The thus produced foamedcement exits foam generator 124 through third leg 130, which is in fluidflow communication with foam capture cell 136, as described below. Insome embodiments, first leg 126 and third leg 130 are parallel andin-line. Generally, the leg arrangements will be configured so as toreflect conditions for producing foam at the well site.

In other embodiments, foam generator 124 may have other configurations,such as a cross configuration where there are four legs. Three of thelegs are used for introduction of cement slurry or gas, and the fourthleg is used to remove the foamed cement from the foam generator.Typically, suitable foam generators are configured to have a zone whereturbulent mixing of the gas and the cement slurry can occur. Further,such foam generators typically utilize a choke as described above forthe tee configuration.

Returning to FIG. 4, third leg 130 is in fluid flow communication withfoam capture cell 136 through conduit 138. Flow through conduit 138 canbe controlled by one or more valves 140. Thus, as illustrated flow outof foam capture cell 136 can be directed to foam capture cell 136 or towaste 142. Additionally, flow out of foam capture cell 136 can beprevented.

As illustrated, foam capture cell 136 comprises a cylinder 144. Foamcapture cell 136 can also comprise a piston in some embodiments;however, generally the use of a piston is not preferred in order to aidin depressurization of captured foamed cement for further testing. Thefoam capture cell is pre-pressurized to a pre-defined foam capturepressure using a pressurized gas from a pump or from another pressurizedgas source 154. The pressure of the foam capture cell 136 serves as abackpressure in apparatus 100 before cement is foamed. The pressurizedgas for backpressure can be air or any other suitable gas. Typically, itwill be nitrogen. The backpressure and flow of gas out of foam capturecell 136 during introduction of foamed cement can be regulated by asuitable backpressure regulator 152, as is known in the art.

Apparatus 100 can use any suitable cylinders for cement reservoir cell110 and foam capture cell 136. Suitable cells for use as the cementreservoir cell and foam capture cell are commercially available. Forexample, the cells can be stainless steel tubular reactors. Reactors areavailable which are double-ended pressure vessels made fromstainless-steel seamless tubing. They can have caps of alloy steel withat least one high-pressure connection at each end. Additionally, theycan utilize piston separators for separating liquid from a gas and othersimilar applications.

As described herein, movement of piston 114 has been described as upwardor downward and cells 110 and 136 have been described as having upperand lower ends; however, such designations are for convenience. Oneskilled in the art will realize that cells 110 and 136 can have otherorientations.

The above-described apparatus is used to prepare foamed cement in alaboratory environment for analysis to determine the type and quality offoamed cement to use in a downhole operation in a well at the well siteor field site. In operation, a base cement slurry is first preparedaccording to design. A surfactant or foaming agent is then injected intothe base cement slurry just before the foam generator. The final foamedcement slurry is created inside the foam generator. The testing isapplicable to most base cement compositions and surfactants, which canbe chosen by one skilled in the art based on the field applications andconditions at the well site.

For example, a variety of hydraulic cements can be utilized in thecement compositions including those comprised of oxides of calcium,aluminum, silicon, iron and/or sulfur which set and harden by reactionwith water. Such hydraulic cements include Portland cements, pozzolanacements, gypsum cements, aluminous cements and silica cements. Portlandcements or their equivalents are generally preferred for use inaccordance with the present invention. Portland cements of the typesdefined and described in the API Specification for Materials and Testingfor Well Cements, API Specification 10, 5th Edition, dated Jul. 1, 1990of the American Petroleum Institute are particularly suitable. PreferredAPI Portland cements include classes A, B, C, G and H with API classes Gand H being the most preferred.

Hydraulic cement will typically be mixed with an aqueous fluid, whichfor example can be fresh water or salt water. The term “salt water” isused herein to mean unsaturated aqueous salt solutions and saturatedaqueous salt solutions including brine and seawater. The water isgenerally present in the cement compositions in an amount sufficient toform a slurry, i.e., an amount in the range of from about 30% to about100% by weight of hydraulic cement in the compositions, more preferablyin an amount in the range of from about 35% to about 60%.

The surfactant can be a suitable foaming and/or foam stabilizingsurfactant. Thus, in the above-described hydraulic cement and water basecement composition, a water-soluble mixture of foaming and foamstabilizing surfactants functions to facilitate foaming of the cementcomposition and to stabilize the foam after it is formed. A non-limitingexample of such a mixture of foaming and foam stabilizing surfactantswhich is preferred for use in accordance with this invention iscomprised of an ethoxylated alcohol ether sulfate surfactant, an alkylor alkene amidopropylbetaine surfactant and an alkyl or alkeneamidopropyldimethylamine oxide surfactant. A preferred such mixture iscomprised of 63.3 parts by weight of the ethoxylated alcohol ethersulfate surfactant, 31.7 parts by weight of the alkyl or alkeneamidopropylbetaine surfactant and 5 parts by weight of the alkyl oralkene amidopropyldimethylamine oxide surfactant. The mixture ofsurfactants is described in detail in U.S. Pat. No. 6,063,738 issued onMay 16, 2000 to Chatterji et al. Based on this disclosure, one trainedin the state-of-the-art can readily extend this invention to aqueoussolution or dispersion of other similar surfactants for the purpose. Themixture of surfactants is generally included in the aqueous solution ordispersion in an amount in the range of from about 2% to about 15% byweight of water therein, more preferably in an amount of about 4% toabout 10%.

After the appropriate cement slurry is prepared, the cement slurry isintroduced into cement reservoir cell 110. Valve 122 can be placed in aclosed position and pump 118 can be activated to place the cement slurryat or near the capture pressure.

The pressure of the compressed gas from gas source 102 is adjusted andchoke 132 is selected to provide for the appropriate pressure and ratefor gas entering first leg 126 of foam generator 124. The pressuredifferential across choke 132 is selected to represent the pressuredifferential used at the well site for which the foamed cement is beingtested. Generally, such pressure differentials are from about 400 psi toabout 1400 psi. More typically, the pressure differential will be in therange of about 500 psi to about 1200 psi. In some applications, thepressure differential will be in the range of from about 500 psi toabout 1000 psi or from about 800 psi to about 1200 psi.

Typically, the nitrogen velocity downstream of choke 132 increases withpressure drop, which supplies more energy during the foaming process.While not wishing to be bound by theory, it is generally believed that agreater pressure drop generates smaller nitrogen bubbles and a morestable foamed cement. While not wishing to be bound by theory, thechoked flow theory indicates that gas velocity reaches sonic speed (ormaximum speed) when the pressure drop across the choke is equal to about90% of the downstream pressure (i.e. capture pressure for apparatus 100,or wellhead pressure for foamed cement production in the field).

Once the cement slurry and gas are ready, valve 122 and valve 140 can beadjusted so that cement slurry is displaced from cement reservoir cell110 into foam generator 124 through second leg 128 such that the cementslurry and gas mix to form a foamed cement that flows into third leg 130and is subsequently introduced into foam capture cell 136. Basically,the pressure in foam capture cell 136 is pre-set to the capture pressureusing pressure source 154 and backpressure regulator 152. The pressuresupplied by pump 118 is greater than the capture pressure. Thus, whenthe valves are appropriately opened to allow fluid flow, the pressuresupplied by pump 118 forces piston 114 downward thus forcing the cementslurry from cement reservoir cell 110 into foam generator 124.Similarly, the pressure differential between the incoming foamed cementand the capture pressure allows foamed cement to flow into foam capturecell 136 in a controlled manner. Typically, pump 118 is run in flow ratecontrol mode.

Foamed cement in foam capture cell 136 is allowed to cure or harden andthen is depressurized and removed from foam capture cell 136. Afterremoval, the foamed cement is analyzed to determine its characteristics,which can include bubble size and distribution, mechanical properties,strength-to-density ratio, etc.

The foam generation and testing can be repeated for different foamqualities and/or different cement slurry compositions. Based on theanalysis, the foamed cement having quality and composition that has beendetermined to be most suitable for use in the subject well is selected.Subsequently, the selected foamed cement is used in a downhole processin the subject well.

EXAMPLES Example 1

A base cement slurry was prepared using 1560.5 g cement mixed with 706.8g water to which 15.11 g of a surfactant was added to achieve the baseslurry density recited in Table 1. The surfactant used was a mixture ofan ethoxylated alcohol ether sulfate, an alkyl or alkyene amidopropylbetaine and an alkyl or alkene amidopropyldimethylamine oxidecommercially available from Halliburton Energy Services, Inc. under thetrade name ZONESEALANT 2000™. The base cement slurry was used to preparefoamed cement slurry in an apparatus according to the above disclosure.Nitrogen was used as the foaming gas. Table 1 shows the conditions offoamed cement production.

Base Slurry Choke Nitrogen Capture Cement Flow Sample Density SizePressure Pressure Rate No. (lbm/gal) (mm) (psi) (psi) (mL/min) 1 15.70.1 1850 500 700

After curing of the resulting foamed cement, density was determinedbased on a sedimentation test similar to API 10B-2, section 12.5. Forthe test, a 2-inch diameter by 4-inch length core was obtained. The coresample was cut into 4 discs, each with a 1-inch thickness. The densityof these discs of foamed cement was determined based on the Archimedesprinciple, i.e. by measuring the weight of the sample in air and theweight of displaced water when the sample is submerged in water. Theaverage density of these samples was 10.7 lbm/gal, suggesting the foamedcement has a foam quality of 32%. The maximum density variation betweendifferent samples was ±0.25 lbm/gal.

Example II

Foamed cement was produced by field equipment using a foam generator(“Field FG 1 and Field FG 2”) and in a Waring blender according to API10B-4 (“Waring blender sample”). The cement foam was produced atapproximately 30% foam quality with similar cement composition for thecement slurry as for Sample 1. Nitrogen was used as the foaming gas withField FG 1 and Field FG 2. Air was the foaming gas with the Waringblender. The gas-bubble size distribution of the resulting foamed cementof Example 1 (Lab FG), Field FG 1, Field FG 2 and the Waring blendersample are shown in FIGS. 6 and 7. It was noticed that Field FG 1 andField FG 2 showed some sample-to-sample variations with some commoncharacteristics: they both contain appreciable amounts of small gasbubbles (≤60 μm); and their gas bubble size distribution curves areasymmetric with the mode smaller than the median. On the other hand, theWaring blender sample contains almost no gas bubbles smaller than 60 μmand its gas bubble size distribution curve is more or less symmetricwith the mode and median approximately the same. The foamed cement ofExample 1 (Lab FG) has similar gas-bubble size distributioncharacteristics as Field FG 1 and Field FG.

Several alternative embodiments will now be set forth to further definethe invention. In one group of embodiments, an apparatus for preparingfoamed cement for laboratory analysis is provided. The apparatuscomprises a source of pressurized gas, a cement reservoir cell, a foamcapture cell and a foam generator. The cement reservoir cell isconfigured to pressurize a cement slurry contained within the cementreservoir cell to a capture pressure. The foam generator can have afirst leg in fluid flow communication with the source of pressurizedgas, a second leg in fluid flow communication with the cement reservoircell, and a third leg in fluid flow communication with the foam capturecell. The foam capture cell is configured to receive pressurized foamedcement from the foam generator.

The first leg and the second leg of the foam generator can beperpendicular. The first leg and the third leg of the foam generator canbe parallel and in-line. Typically, the gas is introduced into the foamgenerator through a choke.

In some embodiments, the foam capture cell has a first cylinder, and theapparatus further comprises a second gas source, which can be used toprovide initial pressure to the foam capture cell. In some of theseembodiments, the foam capture cell can have a piston. In suchembodiments, the backpressure regulator controls the backpressure on afirst side of the piston, and the foamed cement produced in the foamgenerator is introduced into the first cylinder from the foam generatoron a second side of the piston.

In some embodiments, the cement reservoir cell has a second cylinder anda piston. In these embodiments, the apparatus further comprises a pumpsuch that activation of the pump provides pressure on a first side ofthe piston and moves the piston in the second cylinder such that cementslurry on the second side of the piston moves out of the second cylinderand is introduced into the foam generator.

In some of the embodiments, the apparatus further comprising a firstvalve having a first position and a second position. The first positionallowing fluid flow from the cement reservoir cell to the foamgeneration tee, and the second position prevents fluid flow from thecement reservoir cell to the foam generation tee. Also, the apparatuscan further comprise a second valve having an on position and an offposition. The on position allows fluid flow from the foam generation teeto the foam capture cell, and the off position prevents fluid flow fromthe foam generation tee to the foam capture cell.

In other embodiments, a method of preparing foamed cement in alaboratory environment for analysis is provided. The method comprisesthe steps of:

-   -   preparing a cement slurry;    -   introducing the cement slurry into a cement reservoir cell;    -   pressurizing the cement reservoir cell to a pressure equal to        the capture pressure;    -   introduce a gas into a foam generator through a first leg of the        foam generator;    -   displacing the cement slurry from the cement reservoir cell into        the foam generator through a second leg of the foam generator        such that the cement slurry and gas mix to form a foamed cement        that flows into a third leg of the foam generator; and    -   introducing the foamed cement from the third leg into a foam        capture cell.

In the method, the backpressure of the foam capture cell can becontrolled so as to control the introduction of foamed cement into thefoam capture cell. In some of these embodiments, the foam capture cellhas a cylinder, and the apparatus further comprises a second gas source,which provides a backpressure to the cylinder. In some of theseembodiments, the foam capture cell can have a piston, and the step ofintroducing the foamed cement from the third leg into the foam capturecell can comprise introducing the foamed cement into the cylinder so asto displace the piston. The backpressure can be supplied to the foamcapture cell on a side of the piston opposite to the introduction offoamed cement.

In some of the above embodiments, the cement reservoir cell can have acylinder and a piston. The cement slurry can be introduced into thecylinder, and the step of displacing the cement slurry can compriserunning a pump to move the piston within the cylinder so as to displacethe cement slurry from the cylinder.

In some embodiments of the method, the gas is introduced into the foamgenerator through a choke. Additionally, the first leg and the secondleg can be perpendicular. Also, the first leg and the third leg can beparallel and in-line.

In some embodiments, the gas is selected from air, nitrogen andhydrogen. In some of these embodiments, the gas is nitrogen.

In some embodiments, the step of preparing a cement slurry comprises thesteps of mixing an aqueous fluid and cement to form a base cementcomposition, and adding a surfactant to the base cement composition.

In many embodiments, the method further comprises removing the foamedcement from the foam capture cell and analyzing the foamed cement. Insome of these embodiments, the method also comprises selecting a qualityand composition of foamed cement that is determined to be most suitablefor use in a well based on the analysis of the foamed cement, and usingthe selected quality and composition of foamed cement in a downholeprocess for the well.

While compositions and methods are described in terms of “comprising,”“containing,” or “including” various components or steps, thecompositions and methods also can “consist essentially of” or “consistof” the various components and steps. Whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues. Additionally, where the term “about” is used in relation to arange it generally means plus or minus half the last significant figureof the range value, unless context indicates another definition of“about” applies.

Also, the terms in the claims have their plain, ordinary meaning unlessotherwise explicitly and clearly defined by the patentee. Moreover, theindefinite articles “a” or “an”, as used in the claims, are definedherein to mean one or more than one of the elements that it introduces.If there is any conflict in the usages of a word or term in thisspecification and one or more patent(s) or other documents that may beincorporated herein by reference, the definitions that are consistentwith this specification should be adopted.

What is claimed is:
 1. A method of preparing foamed cement in alaboratory environment for analysis comprising: preparing a cementslurry; introducing the cement slurry into a cement reservoir cell;pressurizing the cement reservoir cell to a pressure equal to or greaterthan a capture pressure selected to correspond to the conditions at awell site; introducing a gas into a foam generator through a first legof the foam generator; displacing the cement slurry from the cementreservoir cell into the foam generator through a second leg of the foamgenerator such that the cement slurry and gas mix to form a foamedcement that flows into a third leg of the foam generator; andintroducing the foamed cement from the third leg into a foam capturecell.
 2. The method of claim 1, wherein a backpressure of the foamcapture cell is controlled by a backpressure regulator so as to controlthe introduction of foamed cement.
 3. The method of claim 2, wherein thecement reservoir cell has a cylinder and a piston, the cement slurry isintroduced into the cylinder, and the step of displacing the cementslurry comprises running a pump to move the piston within the cylinderso as to displace the cement slurry from the cylinder.
 4. The method ofclaim 3, wherein the step of preparing a cement slurry comprises thesteps of mixing an aqueous fluid and cement to form a base cementcomposition, and adding a surfactant to the base cement composition. 5.The method of claim 4, wherein the gas is introduced into the foamgenerator through a choke.
 6. The method of claim 5, wherein the firstleg and the second leg are perpendicular.
 7. The method of claim 6,wherein the first leg and the third leg are parallel and in-line.
 8. Themethod of claim 1, wherein the gas is selected from air, nitrogen andhydrogen.
 9. The method of claim 8, wherein the gas is nitrogen.
 10. Themethod of claim 1, wherein the step of preparing a cement slurrycomprises the steps of mixing an aqueous fluid and cement to form a basecement composition, and adding a surfactant to the base cementcomposition.
 11. A method of preparing foamed cement in a laboratoryenvironment for analysis comprising: preparing a cement slurry;introducing the cement slurry into a cement reservoir cell; pressurizingthe cement reservoir cell to a pressure equal to or greater than acapture pressure selected to correspond to the conditions at a wellsite; introducing a gas into a foam generator through a first leg of thefoam generator; displacing the cement slurry from the cement reservoircell into the foam generator through a second leg of the foam generatorsuch that the cement slurry and gas mix to form a foamed cement thatflows into a third leg of the foam generator; introducing the foamedcement from the third leg into a foam capture cell; and removing thefoamed cement from the foam capture cell and analyzing the foamedcement.
 12. The method of claim 11, further comprising: selecting aquality and composition of foamed cement that is determined to be mostsuitable for use in a well based on the analysis of the foamed cement,and using the selected quality and composition of foamed cement in adownhole process for the well.
 13. A method of preparing foamed cementin a laboratory environment for analysis comprising: preparing a cementslurry, wherein the step of preparing a cement slurry comprises thesteps of mixing an aqueous fluid and cement to form a base cementcomposition, and adding a surfactant to the base cement composition;introducing the cement slurry into a cement reservoir cell; pressurizingthe cement reservoir cell to a pressure equal to or greater than acapture pressure selected to correspond to the conditions at a wellsite; introducing a gas into a foam generator through a first leg of thefoam generator; displacing the cement slurry from the cement reservoircell into the foam generator through a second leg of the foam generatorsuch that the cement slurry and gas mix to form a foamed cement thatflows into a third leg of the foam generator; and introducing the foamedcement from the third leg into a foam capture cell, wherein abackpressure of the foam capture cell is controlled by a backpressureregulator so as to control the introduction of foamed cement.
 14. Themethod of claim 13, further comprising removing the foamed cement fromthe foam capture cell and analyzing the foamed cement, and wherein thecement reservoir cell has a cylinder and a piston, the cement slurry isintroduced into the cylinder, and the step of displacing the cementslurry comprises running a pump to move the piston within the cylinderso as to displace the cement slurry from the cylinder.
 15. The method ofclaim 14, wherein the gas is introduced into the foam generator througha choke.
 16. A method of preparing foamed cement in a laboratoryenvironment for analysis comprising: preparing a cement slurry, whereinthe step of preparing a cement slurry comprises the steps of mixing anaqueous fluid and cement to form a base cement composition, and adding asurfactant to the base cement composition; introducing the cement slurryinto a cement reservoir cell; pressurizing the cement reservoir cell toa pressure equal to or greater than a capture pressure selected tocorrespond to the conditions at a well site; introducing a gas into afoam generator through a first leg of the foam generator, wherein thegas is introduced into the foam generator through a choke; displacingthe cement slurry from the cement reservoir cell into the foam generatorthrough a second leg of the foam generator such that the cement slurryand gas mix to form a foamed cement that flows into a third leg of thefoam generator, wherein the cement reservoir cell has a cylinder and apiston, the cement slurry is introduced into the cylinder, and the stepof displacing the cement slurry comprises running a pump to move thepiston within the cylinder so as to displace the cement slurry from thecylinder; and introducing the foamed cement from the third leg into afoam capture cell, wherein a backpressure of the foam capture cell iscontrolled by a backpressure regulator so as to control the introductionof foamed cement; and wherein the gas is selected from air, nitrogen andhydrogen.
 17. The method of claim 16, wherein the first leg and thesecond leg are perpendicular.
 18. The method of claim 17, wherein thefirst leg and the third leg are parallel and in-line.
 19. The method ofclaim 16, further comprising removing the foamed cement from the foamcapture cell and analyzing the foamed cement.
 20. The method of claim19, further comprising: selecting a quality and composition of foamedcement that is determined to be most suitable for use in a well based onthe analysis of the foamed cement, and using the selected quality andcomposition of foamed cement in a downhole process for the well.